Trunk piston engines typically operate using various types and qualities of diesel fuels and heavy fuel oils. When heavy fuel oils and conventional lubricant oil compositions mix in different temperature regions of trunk piston engines, however, black sludge (such as asphaltene deposits or other deposits) and other asphaltene derived deposits (such as undercrown deposits) tends to form. Such black sludge or deposit formation can adversely affect the service interval and maintenance cost of trunk piston engines.
Several attempts have been made to develop a lubricating oil composition having improved performance within trunk piston engines operating on heavy fuel oils. For example, EP 1154012 discusses a dispersant-free lubricating oil composition comprising an oil of lubricating viscosity, an overbased metal detergent, and an antiwear additive, wherein the composition can contain small amounts of a dispersant provided that the composition does not substantially demonstrate the dispersancy effect of the component. Similarly, EP 1209218 discusses a dispersant-free lubricating oil composition comprising an oil of lubricating viscosity, an overbased metal detergent, and an antiwear additive, where the composition can contain less than or equal to 1 mass % of a dispersant.
A need still remains, however, for an improved trunk piston engine lubricating oil composition having a Group I base oil that both reduces black sludge formation in trunk piston engines using heavy fuel oil, and which is viscosity-stabilized such that it is resistant to oxidation-based viscosity increase.